Oil sand containing bitumen mined from the ground is generally slurried with a solvent such as water as part of an initial process for eventually removing the bitumen from the oil sand. Oil sand is a type of bitumen deposit typically containing sand, water and very viscous oil (the bitumen). When the oil sand deposit is located relatively close below the ground surface, the oil sand is often extracted from the deposit by mining. The oil sand is mined by excavating down through the ground surface to where the oil sand deposit occurs and removing oil sand from the deposit with heavy machinery.
Typically, this removal of the oil sand from the deposit is done with some of the largest power shovels and dump trucks in the world, with the power shovels removing shovel-loads of oil sand from the deposit and loading the collected oil sand onto conveyors to be carried away for further processing.
The viscous bitumen tends to hold the sand and water together causing the mined oil sand to contain lumps and chunks, some of which can be quite large. Because of the size of some of these pieces of mined oil sand, the mined oil sand is typically “pre-crushed” by running it through a preliminary crusher to crush the pieces of oil sand to a suitable size for transport on a conveyor (i.e. conveyable size).
The pre-crushed oil sand is then transported by conveyor to a slurry preparation unit as known in the art where the pre-crushed oil sand is further processed to form an oil sand and water slurry.
The slurry preparation unit has to ensure that the pieces of oil sand in the oil sand and water slurry are of pumpable size before the slurry is directed to a pump box and pump to be pumped to the next step in its processing, for example, hydrotransporting the slurry in a pipeline for further conditioning. Therefore, oversize pieces of oil sand or other materials have to be prevented from being directed to the pump in order to obtain a pumpable, pipelinable oil sand slurry. There are at least two forms of slurry preparation units that have been used to form the oil sand and water slurry; slurry preparation units that use screening and more recent screen-less slurry preparation units.
Slurry preparation units that use screening typically comprise a vertically stacked series of components. The pre-crushed oil sand is initially fed into a mixing box where water is mixed with the oil sand to form the slurry. From the mixing box, the oil sand and water slurry is passed through some sort of screening device to remove oversize from the oil sand and water slurry. The slurry that passes through the screening device passes into a pump box where it is pumped to the next stage of the process. The rejected oversize that does not pass through the screening device is rerouted to a crusher to be comminuted and then added to a secondary mix box and again mixed with water to form a slurry before this slurry is passed through another screening device. The portion of the slurry that passes through this other screening device is then returned to the main slurry components. The oversize rejects that do not pass through the second screening device are treated as rejects and removed from the system. The removed rejects are typically eventually hauled away by trucks and dumped in a discard area.
Screening devices commonly used in the industry include fixed screen devices; vibrating screen devices; and rotating screen devices. Fixed screen devices are simply one or more fixed screens that the slurry is pored through. They have the advantage of having a relatively high reliability because they do not have as many moving components as other screening device; however, they have lower efficiencies and tend to have higher rejects rates. Vibrating screens typically have a lower reject rate because the movement of the screens allows more material to pass through, however, because of their motion they tend to have lower reliability. Rotating screens can potentially have higher reliability and efficiency than vibrating screens, however, they are very complex requiring an extensive structure and typically have a lower throughput than vibrating screens.
Slurry preparation units that use screens have a disadvantage in that a portion of the oil sand passing through the slurry preparation units is rejected by the system. This rejection of a portion of the oil sand means that the bitumen in this rejected oil sand is lost, as it is not extracted at later process stages like the rest of the system. In some screening processes, the rejection rate can be as high as 8%. This rejection rate can add up to a significant amount of bitumen that is simply being thrown away. More recently, screen-less slurry preparation towers have been used such as the screen-less system described in U.S. Pat. No. 7,431,830.
Screen-less slurry preparation towers form all of the oil sand and other materials entering the slurry preparation tower into a slurry and as such avoid rejects. In particular, essentially all of the oil sand that enters the tower is typically comminuted in one or more stages to a pumpable size while water is being added to it to form a slurry. This allows bitumen to be extracted from essentially all of the oil sand delivered to the slurry preparation tower, thereby essentially eliminating rejects.
Occasionally, however, there may be instances where tramp metal inclusions in mined oil sand may pose a problem for these screen-less slurry preparation towers. Tramp metal is often a piece of metal from machinery used earlier in the process, such as a piece of shovel tooth from the power shovel or a piece of crusher tooth from the primary crusher. If this piece of tramp metal is large enough, when it is fed into the slurry preparation tower along with a portion of oil sand, the tramp metal may damage or even jam one of the roll crushers used in the slurry preparation tower. This may result in the entire process being stopped while the crusher rolls are either repaired or the jam is located and the tramp metal removed. This may lead to lengthy outages to remove the object from the crusher rolls and affect repairs if any damage has occurred.
The prior art screening processes will typically remove the tramp metal through the screening apparatus, However, with screen-less slurry preparation processes, it may be desirable to remove the tramp metal prior to crushing in the slurry preparation tower to avoid such outages.